1. Field
Example embodiments generally relate to mechanical connections and methods. Example embodiments also relate to nuclear plants and to mechanical connections and methods for repairing piping within reactor pressure vessels of the nuclear plants.
2. Description of Related Art
A reactor pressure vessel (“RPV”) of a boiling water reactor (“BWR”) may have a generally cylindrical shape and/or may be closed at both ends (e.g., by a bottom head and a removable top head). A core shroud, or shroud, may surround the reactor core and may be supported by a shroud support structure.
BWRs have numerous piping systems, and such piping systems may be utilized, for example, to transport water throughout the RPV. For example, core spray piping may be used to deliver water from outside the RPV to core spray spargers inside the RPV. The core spray piping and/or spargers may deliver water flow to the reactor core.
In the event of a reactor plant casualty, such as a loss of coolant accident (“LOCA”), cooling water may be delivered to the reactor core through a core spray distribution header that may include a horizontal section and/or a vertical section. The vertical section may be referred to as a downcomer pipe. Water from the downcomer pipe may flow to sparger distribution header pipes inside the RPV via, for example, a sparger T-box.
If required, repair of piping between the downcomer pipe and the distribution headers may include the use of a coupling apparatus. The coupling apparatus also may be used in the event of full piping replacement. Related art coupling apparatuses are discussed, for example, in U.S. Pat. No. 5,947,529 to Jensen (“the '529 patent”) and U.S. Pat. No. 6,131,962 to Jensen et al. (“the '962 patent”). The disclosures of the '529 patent and the '962 patent are incorporated in this application by reference in their entirety.
Intergranular stress corrosion cracking (“IGSCC”) is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high temperature water. The reactor components may be subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and/or other sources such as residual stresses from welding, cold working, and other inhomogeneous metal treatments. In addition, water chemistry, welding, heat treatment, and/or radiation may increase the susceptibility of metal in a component to IGSCC.
Conditions may exist in the reactor that contribute to IGSCC of the core spray piping. One area of susceptibility in the core spray piping may be the welded joints between the sparger T-box and its associated distribution headers. The sparger T-box may be the junction where the core spray downcomer pipe penetrates the shroud and/or branches to sparger distribution header pipes. Specifically, the sparger T-box may be a straight section of pipe capped by a flat plate welded to the end of the pipe. Two sparger pipes may be welded to the sparger T-box, forming a piping tee. These three welded joints may be susceptible to cracking and, in the event that through-wall circumferential cracking should occur at these welded joints, unpredictable leakage may occur.
Another area of concern may be to ensure that the core spray system may prevent excessive fuel clad temperature in the event of a LOCA by delivering cooling water to the core region of the reactor. In the event that through-wall circumferential cracking should occur at these welded joints, the system may be compromised.
In order to prevent unacceptable leakage and to ensure that the core spray system delivers the necessary volumetric flow rate to the reactor core, it may be desirable to provide a clamping system to ensure structural integrity of the sparger T-box and/or to hold the welded joints together in the event that one or more of these welds fail.
Related art clamping systems are discussed, for example, in U.S. Pat. No. 6,456,682 B1 to Jensen (“the '682 patent”) and U.S. Pat. No. 7,724,863 B2 to Jensen (“the '863 patent”), as well as U.S. Patent Publication No. 2010/0246744 A1 to Wroblewski et al. (“the '744 publication”). The disclosures of the '682 patent, the '863 patent, and the '744 publication are also incorporated in this application by reference in their entirety.